The fluid catalytic cracking process has reached a preeminent position in the petroleum industry in the United States for the production of gasoline and middle distillate boiling range products from higher boiling feeds such as vacuum gas oils. The essentials of the process are well defined and described, for example, in "Fluid Catalytic Cracking with Zeolite Catalysts," Venuto et al, Marcel Dekker New York, 1979, Modern Petroleum Technology, Ed. G. D. Hobson, Applied Science Publishers Ltd. 1973, to which reference is made for a general description of the FCC process. Most modern cracking units employ riser cracking in which the high boiling feed which is to be cracked is contacted with hot cracking catalyst in a vertical pipe or riser, after which the vaporous cracking products are separated from the spent cracking catalyst in a reactor vessel which surrounds the top of cracking riser. The separated catalyst is then passed to a regenerator in which the coke (a hydrocarbonaceous material which accumulates on the catalyst during the cracking) is oxidatively removed by contact with an oxygen-containing gas, usually air. Regeneration not only removes the coke from the catalyst but also provides heat from the endothermic cracking reactions; the hot, regenerated catalyst is returned from the regenerator vessel to the foot of the cracking riser for contact with additional cracking feed.
Separation of the catalyst from the cracking vapors is usually effected in one or more cyclone separators within the reactor vessel. The separator cyclones may be connected directly to the riser in order to effect an initial rapid separation of the cracking products from the spent catalyst and additional separators may be provided for separating catalyst from the vapors in the reactor. Cyclone arrangements are shown, for example, in U.S. Pat. No. 4,502,947 and 4,654,060 including an arrangement with a conduit connecting the riser cyclone to the reactor vessel cyclone so as to minimize contact between the cracking products and the catalyst in the reactor vessel. Reference is made to these patents for a disclosure of such cyclone arrangements. As shown there, the cyclones have dependent, vertical diplegs which extend down from the main body of the cyclone towards a dense, fluidized bed of the catalyst at the bottom of the reactor vessel. In order to provide the appropriate pressure differentials within the system, the diplegs may need to be sealed at the bottom. This may be done either by immersing the lower ends of the cyclone diplegs in the dense bed of catalyst or by employing a seal part, as shown in U.S. Pat. No. 4,502,947 and 4,654,060. Another alternative is to provide a flapper valve at the bottom of the dipleg, as shown, for example, in U.S. Pat. No. 3,785,962 (Conner et al) and U.S. Pat. No. 4,606,814 (Haddad).
Flapper valves used in FCCU are generally quite simple mechanical structures employing a horizontally hinged, flat metal plate which is biased towards a closed position at the bottom of the dipleg. Biasing is normally provided by means of a counterweight on the opposite side of the hinge from the valve plate. In addition, the pressure differential between the inside and outside of the diplegs normally helps to keep the valve closed. When the valve is in its normal closed position, catalyst accumulates in the dipleg of the cyclone until it reaches a predetermined height in the dipleg when the weight of the catalyst above the valve overcomes the biasing effect holding the valve closed so that the valve opens and releases catalyst from the dipleg. Initially, the flapper valve is held open by the flow of catalyst but eventually the biasing overcomes this resistance and the valve closes again until the catalyst builds up once again in the dipleg and forces the valve open. Proper functioning in the flapper valve is desirable at all times but especially during startup when it is necessary to establish the requisite pressure differentials in the system for feed, catalyst and product flow to take place. A flapper valve is described in U.S. Pat. No. 4,446,107 (Buyan) to which reference is made for a description of such a valve.
Existing flapper valves, by reason of their simple construction intended to withstand to stand the rigors of operation in the FCCU have given rise to certain problems. The catalyst is a relatively abrasive substance which, at the high temperatures encountered in the FCCU, tends to abrade and erode mechanical parts fairly rapidly unless adequate care is taken during design and fabrication. In any event, wear usually occurs on surfaces including the flapper valve and on the pivot pin about which the hinging frequently occurs. There is therefore, a continued need for improving the operation of FCCU flapper valves.